1. Field of the Invention
The present invention relates to a hydrophilic-hydrophobic transformable composite film and, more particularly, to a hydrophilic-hydrophobic transformable composite film switched by UV light illumination.
2. Description of Related Art
In many situations, the ability to control the surface wettability of solid substrate is very important. In 1972, Fujishima and Honda constructed an electrochemical cell in which a TiO2 electrode was connected with a platinum electrode through an external circuit. When the surface of the TiO2 electrode was irradiated, current flowed from the platinum electrode to the TiO2 electrode through the external circuit. The direction of the current reveals that the oxidation reaction occurs at the TiO2 electrode and the reduction reaction occurs at the platinum electrode. Thereby, Fujishima and Honda suggested that even without the application of any external voltage, water could be decomposed into hydrogen and oxygen by visible light (A. Fujishima and K. Honda, Nature, 238 (1972) 37-38). Since then, significant amounts of research involved with solar energy transformation, and photochemical catalysis, synthesis, and degradation have been widely performed.
Anatase titanium oxide has many advantages such as excellent light-catalysis activity, UV light-excited catalysis activity, self-cleaning property, anti-bacterial property, anti-fogging property, and super-hydrophilicity and therefore has been applied into many types of sanitary equipment manufacture. Among these properties, the photocatalytic properties result from the promotion of electrons (on the surfaces of TiO2 layer) from the valance band to the conduction band under the influence of ultraviolet (UV) or near-UV radiation. Then the hydroxyl free radical or oxygen free radical (O2−) generated may further react with organic matters or water and undergo a series of photocatalytic reactions.
The super-hydrophilicity results from the repulsion between the excited electrons and the oxygen atoms locating on the outer surface of the TiO2 layer. An abundance of oxygen-holes is created after the leaving of oxygen atoms, and following with the refilling of the oxygen-holes with the water molecules. The hydrogen atoms of those water molecules bind to the oxygen atoms of TiO2 molecules having strong reducing activity and therefore a large amount of hydroxyl group forms on the TiO2 layer. Consequently, a TiO2 layer with super-hydrophilicity on the surface is formed.
Anatase titanium oxide is usually in a form of nano particles, which need to be immobilized before practical usage. Accordingly, reactivity and durability should be considered for the immobilization process. When an organic substrate is used, the decomposition of organic substrate itself should be concerned first. Meanwhile, if an inorganic substrate is used, the surfaces of the photo-catalyst may be shielded, thus, results in the lowering of the catalytic efficiency. Therefore, the immobilizing method of anatase titanium oxide is still a challenge to those skilled in this art and thus is of particular interest to researchers.
The TiO2 coating layer applied to the fabrication of commercial products such as sanitary equipment usually has only single hydrophilic property, either hydrophilic or super-hydrophilic, but does not has hydrophilic-hydrophobic transformation ability, and therefore is limited in many application situations. For example, the cleaning efficiency cannot be largely improved if only hydrophilic property appears in the outer coating layer of the sanitary equipment.
Hence, it is desirable to provide an improved hydrophilic-hydrophobic transformable composite film that the hydrophilic or hydrophobic property thereof can be changed by a simple step, and can be applied into the fabrication of several kinds of apparatus such as sanitary equipment, vehicle outer shells, or exterior wall tiles, and this improves the cleaning efficiency of those applied usages.